Author: Francesca Bulian (Universidad de Salamanca, Spain)
In our blog, Federico already tackled the amazing concept of a disappearing sea, emphasizing how rising of new mountain chains and consequent lend-sea redistribution impacts marine basins. These geological changes can happen on different time scales, from less than a million up to hundreds of millions of years (Ma), and when talking about the Messinian Salinity Crises is most certainly crucial to understand how the Tethys Sea evolved towards the present-day Mediterranean. In the Late Oligocene (27-23 Ma), the Tethys was a large sea, at the east made up by the current Mediterranean connected by a west strand to the Indian Ocean and Paratethys Seas, comprising the present-day Black and Caspian Sea (Zhang et al., 2014). Starting from the Tortonian (Late Miocene), the Tethys become significantly smaller, when its west branch become replaced by the Arabian peninsula (Barrier and Vrielynck, 2008).
The restriction of this water mass not only changed the proto-Mediterranean paleogeography but deeply impacted the climate as well. In fact, the Tethys shrinking, drastically weakened the African Summer Monsoon and caused a reduction in precipitation above the north African continent and consequent aridification of the area. Basically, when a large part of the Tethys was replaced by land, the sea-surface heat fluxes decreased, while the surface of hot exposed land become bigger. Because of his energetical disbalance less moisture was able to be transported from the global ocean resulting in less rain in north of Africa (Zhang et al., 2014). In addition to changes in mean climate which become generally more arid, north Africa become more sensitive to orbital forcing as well making possible occasional changes towards greener and wetter Sahara landscape as well.
Different studies, like Schuster (2006), actually found eolian (produced by wind) dune deposits in the present-day Sahara desert region, covering 2000 km2 which were dated at 7 Ma. This could imply that the Tethys restriction and consequent aridification of the Northern African region lead to the formation of a first proto-Sahara Desert. Records from the lake Chad from that time onwards, show some more humid phases as well, characterized by lake level growth which could be the result of the increased climate sensitivity, even if within a global increasingly arid climate (Novello et al., 2015). We saw how the redistribution of land masses in the Mediterranean region, already from 7 Ma lead to important changes in the local climate, but the north Africa landscape, because of the increased aridity, changed completely too. The widespread rainforests that covered the majority of north Africa were replaced by grasslands and savannah like habitats. These environmental changes affected the biosphere as well and facilitated the appearance of new ungulate mammals with a grass dominated diet (Sepulchre et al., 2006) in addition to the splitting of the mammalian order Macroscelidea (elephant). Initially thought as a result of genus dispersion, the Elephantulus rozeti species, endemic of the northern Sahara Desert, split from the sister group of species from a different genus Petrodromus tetradactylus, exclusive of the southern Sahara, was most recently attributed to this climatic change (Douady et al., 2003). But why such drastic changes? Well, the Sahara aridification, paired with analogous settings happening in other parts of the world as well like Asia, played an important role in the global climate. Actually, on a global scale the Late Miocene is characterized by evident climate cooling which has been recorded in core sediments all over the world and confirmed by Sea Surface Temperature reconstructions (Herbert et al., 2016).
Can the aridification of some parts of the globe like northern Africa and Asia result in global cooling? According to some scientists the two can actually be strongly related, particularly if we focus on the carbon cycle. A global cooling could mean a global drop in CO2 concentrations (opposite of what is happening today) that could be also caused by the Late Miocene spreading of grasslands (C4 photosynthesizing plants) at the expense of forests (C3 photosynthesizing plants). This change in distribution of Carbon, could possibly bring long-term changes in the oceanic and terrestrial carbon reservoirs, changing consequently the CO2 concentrations (Holbourn et al., 2018). Even if still quite debated, this theory is just a possibility which illustrates very nicely how every realm/sphere on our planet is deeply connected and a small change or perturbation in one of them could influence deeply the remaining ones.
Finally, it is widely believed that these important climatic changes which caused shifts in African but also Asian flora and fauna (Sahara here was an example) are linked with the emergence of early hominins in north Africa as well (Brunet et al., 2002). In 1994, in the Northern Chad area, new hominid fossil remains have been found. Researchers named the specimen Sahelanthropus tchadensis (Brunet et al., 2002) and gave him an age of 7 Ma (Late Miocene). Because of several cranial derived hominid features, Sahelanthropus is considered to be the oldest and most primitive known member of the hominid clade, positioned in the proximity of the divergence between hominids and chimpanzees. According to Shultz and Maslin (2013), the new climate, that in general terms can be defined as trending towards increasing aridity but still punctuated by periods of high rainfall with the periodic appearance of deep freshwater lakes, were the trigger of hominin speciation and dispersal events.
Coming back to our main topic, the Messinian Salinity Crises (MSC), interesting to note is that exactly around the same time of all these global changes (cca. 7 Ma), we have the first signals of gateway restriction caused by the Betic and/or Rifian corridors uplift. Having then all the above-mentioned possible mechanisms and complex feedbacks in mind, and some recent publications on the topic (Capella et al., 2019), the possible contribution of the changing Atlantic-Mediterranean gateway towards the Miocene global cooling is certainly to be considered more into detail.
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Cover image: Present-day Sahara desert photo.